1. Field of the Invention
This invention relates to a method of and system for carrying out an image processing such as a processing for enhancing a predetermined frequency component of an image signal. This invention further relates to a computer-readable recording medium loaded with program for causing a computer to perform the image processing in accordance with the method.
2. Description of the Related Art
In the field of image processing, a wavelet transformation or a Laplacian pyramid is employed as a method of dividing an image signal into a plurality of frequency components, for instance, when different processings are to be carried out by frequency bands of the image signal. The image processing may be, for instance, separation of a high frequency component for removing noise, or an image compression processing by deleting data in a frequency band where lots of noise exists. We have proposed various image processing methods such as for enhancing only edge components of an image by wavelet transformation. See, for instance, Japanese Unexamined Patent Publication Nos. 6(1994)-274615 and 6(1994)-350989.
The method called xe2x80x9cLaplacian pyramidxe2x80x9d is disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 5(1993)-244508, 6(1994)-96200 and 6(1994)-301766. In the Laplacian pyramid, a mask processing is carried out on an original image by use of a mask approximated by a Gaussian function, and picture elements are thinned by sampling the processed image signal, whereby a quarter-size unsharp image which is xc2xc of the original image in size is obtained. Then the quarter-size unsharp image is interpolated with picture elements whose values are 0 in positions corresponding to the thinned picture elements, whereby the unsharp image is enlarged to the original size, and a mask processing is carried out on the original-size unsharp image by use of the aforesaid mask, whereby another unsharp image is obtained. The unsharp image is subtracted from the original image and a band-limited signal representing a component of the original image signal in a certain limited frequency band, i.e., representing a frequency response characteristic in a certain limited frequency band, is obtained. By repeating these steps with successively obtained unsharp images employed as the original image in sequence, N band-limited signals which are xc2xd2N of the original image in size are made. The unsharp image in the lowest frequency band represents a low frequency component of the original image.
In Japanese Unexamined Patent Publication No. 5(1993)-244508, there is proposed a method in which a processed image signal enhanced with contrast by frequency band is obtained by decomposing a radiation image into a plurality of images in different frequency bands by Laplacian pyramid, thereby obtaining a plurality of band-limited image signals, transforming the band-limited image signals by use of non-linear functions, and reconstructing an image signal from the transformed band-limited image signals together with the unsharp image signal representing the unsharp image in the lowest frequency band.
We have proposed various image processing methods and systems for improving diagnostic performance of a radiation image by carrying out on a radiation image signal representing the radiation image, for instance, a frequency enhancement processing or a dynamic range compression processing by use of an unsharp mask image signal (will be referred to as xe2x80x9cunsharp image signalxe2x80x9d, hereinbelow). See, for instance, Japanese Unexamined Patent Publication Nos. 55(1980)-163472, 55(1980)-87953, 3(1991)-222577, 10(1998)-75395, and 10(1998)-171983. For example, in the frequency enhancement processing, a predetermined spatial frequency component of an original image signal is enhanced by subtracting an unsharp image signal Sus from the original image signal Sorg, and adding the remainder multiplied by a coefficient of enhancement xcex2 to the original image signal Sorg. This is represented by the following formula (1).
Sproc=Sorg+xcex2xc3x97(Sorgxe2x88x92Sus)xe2x80x83xe2x80x83(1)
wherein Sproc is a frequency-enhanced image signal, Sorg is an original image signal, Sus is an unsharp image signal and xcex2 is a coefficient of enhancement.
Further, in Japanese Unexamined Patent Publication No. 10(1998)-75395, there is disclosed a method of preventing generation of an artifact in the frequency-enhanced image signal by adjusting the frequency response characteristic of the add signal to be added to the original image signal. In this method, a plurality of unsharp image signals, which are different from each other in frequency response characteristic, that is, in sharpness, are prepared, differences between two of the original image signal and the unsharp image signals are taken, thereby making a plurality of band-limited signals respectively representing frequency components in limited frequency bands of the original image signal, the band-limited signals thus obtained are transformed into signals of desired values by use of different transformation functions, and the add signal is made by adding up the suppressed band-limited signals. This is represented, for instance, by the following formulae (2).                                                                         S                proc                            =                              xe2x80x83                            ⁢                                                S                  org                                +                                  β                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          S                      org                                        )                                    xc3x97                                      F                    usm                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  S                        org                                            ,                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        1                                            ,                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        2                                            ,                                              …                        ⁢                                                  xe2x80x83                                                ⁢                                                  S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        n                                                              )                                                                                                                                                            F                  usm                                ⁢                                  xe2x80x83                                ⁢                                  (                                                            S                      org                                        ,                                                                  S                        us                                            ⁢                                              xe2x80x83                                            ⁢                      1                                        ,                                                                  S                        us                                            ⁢                                              xe2x80x83                                            ⁢                      2                                        ,                                          …                      ⁢                                              xe2x80x83                                            ⁢                                              S                        us                                            ⁢                                              xe2x80x83                                            ⁢                      n                                                        )                                            =                              xe2x80x83                            ⁢                                                                    f                    1                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  S                        org                                            -                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        1                                                              )                                                  +                                                      f                    2                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                                                                            S                          us                                                ⁢                        1                                            -                                                                        S                          us                                                ⁢                        2                                                              )                                                  +                …                                                                                                        xe2x80x83                            ⁢                                                                    +                                          f                      k                                                        ⁢                                      xe2x80x83                                    ⁢                                      (                                                                                            S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        k                                            -                      1                      -                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        k                                                              )                                                  +                …                +                                                      f                    n                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                                                                            S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        n                                            -                      1                      -                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        n                                                              )                                                                                                          (        2        )            
wherein Sproc is a processed image signal, Sorg is an original image signal, Susk (k=1 to n) is an unsharp image signal, fk(k=1 to n) is a transformation function, and xcex2(Sorg) is a coefficient of enhancement determined on the basis of the original image signal.
Further, in Japanese Unexamined Patent Publication No. 10(1998)-75364, there is disclosed a method of preventing generation of an artifact in the processed signal when the dynamic range compression processing is to be carried out. In this method, as disclosed in Japanese Unexamined Patent Publication No. 10(1998)-75395, a plurality of band-limited signals are made, a low frequency component signal representing a low frequency component of the original image signal is obtained on the basis of the band-limited signals, and the dynamic range compression processing is carried out by the low frequency component signal to the original image signal. This is represented, for instance, by the following formula (3).                                                                         S                proc                            =                              xe2x80x83                            ⁢                                                S                  org                                +                                  D                  ⁢                                      {                                                                  S                        org                                            -                                                                        F                          drc                                                ⁢                                                  xe2x80x83                                                ⁢                                                  (                                                                                    S                              org                                                        ,                                                                                          S                                us                                                            ⁢                                                              xe2x80x83                                                            ⁢                              1                                                        ,                                                                                          S                                us                                                            ⁢                                                              xe2x80x83                                                            ⁢                              2                                                        ,                                                          …                              ⁢                                                              xe2x80x83                                                            ⁢                                                              S                                us                                                            ⁢                                                              xe2x80x83                                                            ⁢                              n                                                                                )                                                                                      }                                                                                                                                                            F                  drc                                ⁢                                  xe2x80x83                                ⁢                                  (                                                            S                      org                                        ,                                                                  S                        us                                            ⁢                                              xe2x80x83                                            ⁢                      1                                        ,                                                                  S                        us                                            ⁢                                              xe2x80x83                                            ⁢                      2                                        ,                                          …                      ⁢                                              xe2x80x83                                            ⁢                                              S                        us                                            ⁢                                              xe2x80x83                                            ⁢                      n                                                        )                                            =                              xe2x80x83                            ⁢                              {                                                                            f                      d1                                        ⁢                                          xe2x80x83                                        ⁢                                          (                                                                        S                          org                                                -                                                                              S                            us                                                    ⁢                                                      xe2x80x83                                                    ⁢                          1                                                                    )                                                        +                                                            f                      d2                                        ⁢                                          xe2x80x83                                        ⁢                                          (                                                                                                    S                            us                                                    ⁢                                                      xe2x80x83                                                    ⁢                          1                                                -                                                                              S                            us                                                    ⁢                                                      xe2x80x83                                                    ⁢                          2                                                                    )                                                        +                  …                                                                                                                        xe2x80x83                            ⁢                                                                    +                                          f                      dk                                                        ⁢                                      xe2x80x83                                    ⁢                                      (                                                                                            S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        k                                            -                      1                      -                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        k                                                              )                                                  +                …                +                                                      f                    dn                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                                                                            S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        n                                            -                      1                      -                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        n                                                              )                                                                                                          (        3        )            
wherein Sproc is a processed image signal, Sorg is an original image signal, Susk (k=1 to n) is an unsharp image signal, fdk(k=1 to n) is a transformation function for obtaining the low frequency component signal, and D{Sorgxe2x88x92Fdrc(Sorg, Sus1, Sus2, . . . Susn)} is a coefficient of dynamic range compression determined on the basis of the low frequency component signal, D being a function for transforming D{Sorgxe2x88x92Fdrc(Sorg, Sus1, Sus2, . . . Susn)}.
Further, in Japanese Unexamined Patent Publication No. 10(1998)-171983, there is disclosed a method of preventing generation of an artifact in the processed signal when both the frequency enhancement processing and the dynamic range compression processing are to be carried out. In this method, as disclosed in Japanese Unexamined Patent Publication No. 10(1998)-75395, a plurality of band-limited signals are made in the manner described above, a high frequency component signal representing a high frequency component of the original image signal and a low frequency component signal representing a low frequency component of the original image signal are obtained on the basis of the band-limited signals, and the frequency enhancement processing and the dynamic range compression processing are carried out by adding the high frequency component signal and the low frequency component signal to the original image signal. This is represented, for instance, by the following formula (4).                                                                         S                proc                            =                              xe2x80x83                            ⁢                                                S                  org                                +                                  β                  ⁢                                      xe2x80x83                                    ⁢                                                            (                                              S                        org                                            )                                        ·                                          F                      usm                                                        ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  S                        org                                            ,                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        1                                            ,                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        2                                            ,                                              …                        ⁢                                                  xe2x80x83                                                ⁢                                                  S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        n                                                              )                                                  +                                                                                                        xe2x80x83                            ⁢                              D                ⁢                                  {                                                            S                      org                                        -                                                                  F                        drc                                            ⁢                                              xe2x80x83                                            ⁢                                              (                                                                              S                            org                                                    ,                                                                                    S                              us                                                        ⁢                                                          xe2x80x83                                                        ⁢                            1                                                    ,                                                                                    S                              us                                                        ⁢                                                          xe2x80x83                                                        ⁢                            2                                                    ,                                                      …                            ⁢                                                          xe2x80x83                                                        ⁢                                                          S                              us                                                        ⁢                                                          xe2x80x83                                                        ⁢                            n                                                                          )                                                                              }                                                                                                                                          F                  usm                                ⁢                                  xe2x80x83                                ⁢                                  (                                                            S                      org                                        ,                                                                  S                        us                                            ⁢                                              xe2x80x83                                            ⁢                      1                                        ,                                                                  S                        us                                            ⁢                                              xe2x80x83                                            ⁢                      2                                        ,                                          …                      ⁢                                              xe2x80x83                                            ⁢                                              S                        us                                            ⁢                                              xe2x80x83                                            ⁢                      n                                                        )                                            =                              xe2x80x83                            ⁢                              {                                                                            f                      u1                                        ⁢                                          xe2x80x83                                        ⁢                                          (                                                                        S                          org                                                -                                                                              S                            us                                                    ⁢                                                      xe2x80x83                                                    ⁢                          1                                                                    )                                                        +                                                            f                      u2                                        ⁢                                          xe2x80x83                                        ⁢                                          (                                                                                                    S                            us                                                    ⁢                                                      xe2x80x83                                                    ⁢                          1                                                -                                                                              S                            us                                                    ⁢                                                      xe2x80x83                                                    ⁢                          2                                                                    )                                                        +                  …                                                                                                                        xe2x80x83                            ⁢                                                                    +                                          f                      uk                                                        ⁢                                      xe2x80x83                                    ⁢                                      (                                                                                            S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        k                                            -                      1                      -                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        k                                                              )                                                  +                …                +                                                      f                    un                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                                                                            S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        N                                            -                      1                      -                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        N                                                              )                                                                                                                                                            F                  drc                                ⁢                                  xe2x80x83                                ⁢                                  (                                                            S                      org                                        ,                                                                  S                        us                                            ⁢                                              xe2x80x83                                            ⁢                      1                                        ,                                                                  S                        us                                            ⁢                                              xe2x80x83                                            ⁢                      2                                        ,                                          …                      ⁢                                              xe2x80x83                                            ⁢                                              S                        us                                            ⁢                                              xe2x80x83                                            ⁢                      n                                                        )                                            =                              xe2x80x83                            ⁢                              {                                                                            f                      d1                                        ⁢                                          xe2x80x83                                        ⁢                                          (                                                                        S                          org                                                -                                                                              S                            us                                                    ⁢                                                      xe2x80x83                                                    ⁢                          1                                                                    )                                                        +                                                            f                      d2                                        ⁢                                          xe2x80x83                                        ⁢                                          (                                                                                                    S                            us                                                    ⁢                                                      xe2x80x83                                                    ⁢                          1                                                -                                                                              S                            us                                                    ⁢                                                      xe2x80x83                                                    ⁢                          2                                                                    )                                                        +                  …                                                                                                                        xe2x80x83                            ⁢                                                                    +                                          f                      dk                                                        ⁢                                      xe2x80x83                                    ⁢                                      (                                                                                            S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        k                                            -                      1                      -                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        k                                                              )                                                  +                …                +                                                      f                    dn                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                                                                            S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        n                                            -                      1                      -                                                                        S                          us                                                ⁢                                                  xe2x80x83                                                ⁢                        n                                                              )                                                                                                          (        4        )            
wherein Sproc is a processed image signal, Sorg is an original image signal, Susk (k=1 to n) is an unsharp image signal, fuk(k=1 to n) is a transformation function for obtaining the high frequency component signal, fdk(k=1 to n) is a transformation function for obtaining the low frequency component signal, xcex2(Sorg) is a coefficient of enhancement determined on the basis of the original image signal, and D{Sorgxe2x88x92Fdrc(Sorg, Sus1, Sus2, . . . Susn)} is a coefficient of dynamic range compression determined on the basis of the low frequency component signal, D being a function for transforming D{Sorgxe2x88x92Fdrc(Sorg, Sus1, Sus2, . . . Susn)}.
In the frequency enhancement processing and the dynamic range compression processing (will be representatively referred to xe2x80x9cas the transformation processingxe2x80x9d, hereinbelow), the frequency response characteristic of the add signal to be added to the original image signal can be adjusted by changing the definition of the transformation functions and the like for transforming the band-limited signals. Accordingly, a processed image signal having a desired frequency response characteristic, e.g., suitable for preventing generation of an artifact, can be obtained by properly defining the transformation functions.
The unsharp image signals used in the aforesaid transformation processing are made by filtering the original image signal to obtain an image signal with less picture elements, further filtering the image signal with less picture elements, repeating these steps a desired times and interpolating picture elements into the image signal with less picture elements obtained at each filtering stage so that the number of the picture elements in the image signal becomes equal to that in the original image signal. Accordingly, each of the unsharp image signals is an image signal which is the same as the original image signal in the number of picture elements but lower than the original image signal in sharpness.
The band-limited image signals are made, for instance, by taking differences between unsharp image signals in adjacent frequency bands, or differences between the original image signal and the respective unsharp image signals. Accordingly, the band-limited image signals are the same as the original image signal in number of the picture elements and represent the frequency response characteristics of the original image signal for the corresponding frequency bands.
In a radiation image, quantum noise is more conspicuous at a portion which is exposed to less radiation and is low in density. In the aforesaid Japanese Unexamined Patent Publication No. 6(1994)-96200, there is disclosed a method in which band-limited image signals in a plurality of frequency bands are obtained by decomposing a radiation image into images in a plurality of frequency bands by Laplacian pyramid, the local dispersions of the band-limited image signals are calculated, a noise removing processing is carried out on each of the band-limited image signals according to its value of the local dispersion, and a processed image signal suppressed with noise according to the frequency band is obtained by reconstructing an image from the noise-removed band-limited image signals and an unsharp image signal representing an unsharp image in the lowest frequency band.
In the method disclosed in the aforesaid Japanese Unexamined Patent Publication No. 5(1993)-244508, since a processed image signal is obtained by transforming the band-limited image signals by use of non-linear functions and reconstructing an image signal from the transformed band-limited image signals, the band-limited image signals must be retransformed and an image signal must be reconstructed from the retransformed band-limited image signals when the degree of transformation is to be corrected. Accordingly, it takes a long time to correct the degree of transformation. When an optimal image processing is set by variously changing the contents of the image processing while monitoring the result of change of the contents on a CRT, the operator is put under a heavy stress.
In the method disclosed in the aforesaid Japanese Unexamined Patent Publication No. 6(1994)-96200, since the processing of removing noise components is carried out on the band-limited image signals in the respective frequency bands, when removal of noise is excessive or insufficient, it is necessary to carry out the processing of removing noise components on the respective band-limited image signals with the changed degree of removing noise and then to reconstruct an image, and accordingly, it takes a long time to correct the degree of removing noise. When an optimal degree of removing noise is set by variously changing the degree of removing noise while monitoring the result of change on a CRT, the operator is put under a heavy stress.
In view of the foregoing observations and description, the primary object of the present invention is to provide a method of and a system for image processing which can carry out a desired image processing such as a processing for enhancing a particular frequency component using the aforesaid band-limited image signals at a high speed.
Another object of the present invention is to provide a method of and a system for image processing which can properly remove noise from an original image.
Still another object of the present invention is to provide a computer-readable recording medium loaded with program for causing a computer to perform the image processing method in accordance with the method of the present invention.
In accordance with a first aspect of the present invention, there is provided an image processing method for obtaining a processed image signal from an original image signal representing an original image by carrying out on the original image signal an image processing based on a signal representing information on a high frequency component of the original image signal, the method comprising the steps of
making band-limited image signals from the original image signal,
obtaining a signal representing information on a high frequency component of the original image signal from the band-limited image signals on the basis of a predetermined transformation function, and
carrying out said image processing on the original image signal on the basis of the signal representing information on a high frequency component of the original image signal.
The xe2x80x9cimage processingxe2x80x9d includes, for instance, a frequency enhancement processing for enhancing a particular frequency component of the original image signal and a dynamic range compression processing for reducing the contrast of the high density range and/or the low density range, thereby narrowing the dynamic range which is the difference between the maximum density and the minimum density of the original image. Further the image processing may comprise both a frequency enhancement processing and a dynamic range compression processing. It is preferred that the xe2x80x9cpredetermined transformation functionxe2x80x9d be a function selected according to the contents of the image processing.
It is preferred that each of the images represented by the band-limited image signals has picture elements in a number according to the corresponding frequency band.
It is preferred that the signal representing information on a high frequency component of the original image signal has the same number of picture elements as the original image signal. In other words, it is preferred that the image represented by the signal representing information on a high frequency component of the original image signal be the same as the original image in size.
For example, the band-limited image signals may be obtained by carrying out multi-resolution transformation on the original image signal (transforming the original image signal to multiple resolution image signals), and the signal representing information on a high frequency component of the original image signal may be obtained by obtaining transformed band-limited image signals by transforming the band-limited image signals by use of a predetermined transformation function and carrying out inverse multi-resolution transformation on the transformed band-limited image signals. The inverse multi-resolution transformation is a transformation which corresponds to the multi-resolution transformation and is for reconstructing (reversibly or irreversibly) the original signal from the transformed band-limited image signals.
When the band-limited image signals are obtained by carrying out multi-resolution transformation on the original image signal, Laplacian pyramid decomposition may be employed or a method in which the original image signal is transformed to a plurality of image signals representing the frequency response characteristics of the respective frequency bands by wavelet transformation may be employed. When the band-limited image signals are obtained by the Laplacian pyramid decomposition, Laplacian pyramid reconstruction may be employed as the inverse multi-resolution transformation, and when the band-limited image signals are obtained by the wavelet transformation, inverse wavelet transformation may be employed as the inverse multi-resolution transformation.
When the original image signal is transformed to multiple resolution image signals by the Laplacian pyramid decomposition or the wavelet transformation, the image signal in the lowest frequency band represents low frequency information obtained by contracting the original image and is not a band-limited image signal representing the frequency response characteristic. Accordingly, it is preferred that the image signal in the lowest frequency band be not used in the processing or used as 0.
It is preferred that the predetermined transformation function be a nonlinear function.
In accordance with a second aspect of the present invention, there is provided an image processing system for obtaining a processed image signal from an original image signal representing an original image by carrying out on the original image signal an image processing based on a signal representing information on a high frequency component of the original image signal, the system comprising
a band-limited image signal making means which makes band-limited image signals from the original image signal,
a high frequency component obtaining means which obtains a signal representing information on a high frequency component of the original image signal from the band-limited image signals on the basis of a predetermined transformation function, and
an image processing means which carries out said image processing on the original image signal on the basis of the signal representing information on a high frequency component of the original image signal.
The signal representing information on a high frequency component of the original image signal will be referred to as xe2x80x9cthe high frequency component signalxe2x80x9d, hereinbelow.
It is preferred that the band-limited image signal making means makes the band-limited image signals so that each of the images represented by the band-limited image signals has picture elements in a number according to the corresponding frequency band.
It is preferred that the high frequency component obtaining means obtains a high frequency component signal which has the same number of picture elements as the original image signal.
Further, it is preferred that the band-limited image signal making means makes band-limited image signals by carrying out multi-resolution transformation on the original image signal, and the high frequency component obtaining means obtains a high frequency component signal by obtaining transformed band-limited image signals by transforming the band-limited image signals by use of a predetermined transformation function and carrying out inverse multi-resolution transformation on the transformed band-limited image signals.
In this case, the multi-resolution transformation may comprise Laplacian pyramid decomposition or a wavelet transformation.
It is preferred that the high frequency component obtaining means obtains the high frequency component signal from the band-limited image signals other than that in the lowest frequency band.
It is preferred that the predetermined transformation function be a nonlinear function.
It is preferred that the image processing comprises a frequency enhancement processing and/or a dynamic range compression processing.
In accordance with a third aspect of the present invention, there is provided a computer-readable recording medium loaded with program for causing a computer to perform the image processing method in accordance with the first aspect of the present invention.
In accordance with the first to third aspects of the present invention, the degree of image processing on the original image signal can be freely changed by only changing the level of the high frequency component signal, and accordingly, the degree of image processing can be easily changed in a shorter time as compared with the method disclosed in Japanese Unexamined Patent Publication No. 5(1993)-244508 where the degree of image processing is changed by correcting the nonlinear functions, whereby the time required to obtain a processed image signal changed with the degree of image processing is shortened and stress on the operator can be lightened.
When each of the images represented by the band-limited image signals has picture elements in a number according to the corresponding frequency band, the amount of operation for transformation processing can be reduced and the time required to obtain a processed image signal is further shortened.
Further, when the high frequency component signal (the signal representing information on a high frequency component of the original image signal) has the same number of picture elements as the original image signal, the image processing can be carried out on the original image signal without changing the size of the high frequency component signal, and accordingly the image processing can be carried out more efficiently.
In accordance with a fourth aspect of the present invention, there is provided an image processing method for obtaining a processed image signal from an original image signal representing an original image by carrying out on the original image signal an image processing based on a signal representing information on a high frequency component of the original image signal, the method comprising the steps of
making band-limited image signals from the original image signal,
obtaining noiseless band-limited image signals by removing a noise component from the band-limited image signals,
obtaining a signal representing information on a high frequency component of the original image signal from the noiseless band-limited image signals on the basis of a predetermined transformation function, and
carrying out said image processing on the original image signal on the basis of the signal representing information on a high frequency component of the original image signal.
The xe2x80x9cimage processingxe2x80x9d includes, for instance, a frequency enhancement processing for enhancing a particular frequency component of the original image signal and a dynamic range compression processing for reducing the contrast of the high density range and/or the low density range, thereby narrowing the dynamic range which is the difference between the maximum density and the minimum density of the original image. Further the image processing may comprise both a frequency enhancement processing and a dynamic range compression processing. It is preferred that the xe2x80x9cpredetermined transformation functionxe2x80x9d be a function selected according to the contents of the image processing.
The expression xe2x80x9cremoving a noise component from the band-limited image signalsxe2x80x9d means to reduce the noise component included in the band-limited image signals, and xe2x80x9cremoving a noise component from the band-limited image signalsxe2x80x9d may be performed by, for instance, subtracting from each of the band-limited image signals its noise component or its noise component multiplied by a coefficient representing a desired noise suppression level. The coefficient may be changed depending on the value of the band-limited image signal.
It is preferred that each of the images represented by the band-limited image signals has picture elements in a number according to the corresponding frequency band.
It is preferred that the signal representing information on a high frequency component of the original image signal has the same number of picture elements as the original image signal.
That the signal representing information on a high frequency component of the original image signal has the same number of picture elements as the original image signal means that the image represented by the signal representing information on a high frequency component of the original image signal is the same as the original image in size.
For example, the band-limited image signals may be obtained by carrying out multi-resolution transformation on the original image signal (transforming the original image signal to multiple resolution image signals), and the signal representing information on a high frequency component of the original image signal may be obtained by obtaining transformed noiseless band-limited image signals by transforming the noiseless band-limited image signals by use of a predetermined transformation function and carrying out inverse multi-resolution transformation on the transformed noiseless band-limited image signals. The inverse multi-resolution transformation is a transformation which corresponds to the multi-resolution transformation and is for reconstructing (reversibly or irreversibly) the original signal from the transformed noiseless band-limited image signals.
When the band-limited image signals are obtained by carrying out multi-resolution transformation on the original image signal, Laplacian pyramid decomposition may be employed or a method in which the original image signal is transformed to a plurality of image signals representing the frequency response characteristics of the respective frequency bands by wavelet transformation may be employed. When the band-limited image signals are obtained by the Laplacian pyramid decomposition, Laplacian pyramid reconstruction may be employed as the inverse multi-resolution transformation, and when the band-limited image signals are obtained by the wavelet transformation, inverse wavelet transformation may be employed as the inverse multi-resolution transformation.
When the original image signal is transformed to multiple resolution image signals by the Laplacian pyramid decomposition or the wavelet transformation, the image signal in the lowest frequency band represents low frequency information obtained by contracting the original image and is not a band-limited image signal representing the frequency response characteristic. Accordingly, it is preferred that the image signal in the lowest frequency band be not used in the processing or used as 0.
It is preferred that the predetermined transformation function be a nonlinear function.
Further it is preferred that the noise component be removed from the band-limited image signals by filtering processing by an iris filter.
In accordance with a fifth aspect of the present invention, there is provided an image processing system for obtaining a processed image signal from an original image signal representing an original image by carrying out on the original image signal an image processing based on a signal representing information on a high frequency component of the original image signal, the system comprising
a band-limited image signal making means which makes band-limited image signals from the original image signal,
a noiseless band-limited image signal obtaining means which obtains noiseless band-limited image signals by removing a noise component from the band-limited image signals,
a high frequency component obtaining means which obtains a signal representing information on a high frequency component of the original image signal from the noiseless band-limited image signals on the basis of a predetermined transformation function, and
an image processing means which carries out said image processing on the original image signal on the basis of the signal representing information on a high frequency component of the original image signal.
The signal representing information on a high frequency component of the original image signal will be referred to as xe2x80x9cthe high frequency component signalxe2x80x9d, hereinbelow.
It is preferred that the band-limited image signal making means makes the band-limited image signals so that each of the images represented by the band-limited image signals has picture elements in a number according to the corresponding frequency band.
It is preferred that the high frequency component obtaining means obtains a high frequency component signal which has the same number of picture elements as the original image signal.
Further, it is preferred that the band-limited image signal making means makes band-limited image signals by carrying out multi-resolution transformation on the original image signal, and the high frequency component obtaining means obtains a high frequency component signal by obtaining transformed noiseless band-limited image signals by transforming the noiseless band-limited image signals by use of a predetermined transformation function and carrying out inverse multi-resolution transformation on the transformed noiseless band-limited image signals.
In this case, the multi-resolution transformation may comprise Laplacian pyramid decomposition or a wavelet transformation.
It is preferred that the high frequency component obtaining means obtains the high frequency component signal from the noiseless band-limited image signals obtained from the band-limited image signals other than that in the lowest frequency band.
It is preferred that the predetermined transformation function be a nonlinear function.
It is preferred that the image processing comprises a frequency enhancement processing and/or a dynamic range compression processing.
Further it is preferred that the noiseless band-limited image signal obtaining means removes the noise component from the band-limited image signals by filtering processing by an iris filter.
In accordance with a sixth aspect of the present invention, there is provided a computer-readable recording medium loaded with program for causing a computer to perform the image processing method in accordance with the fourth aspect of the present invention.
In accordance with the fourth to sixth aspects of the present invention, the degree of image processing on the original image signal can be freely changed by only changing the level of the high frequency component signal, and accordingly, the degree of image processing can be easily changed in a shorter time as compared with the method disclosed in Japanese Unexamined Patent Publication No. 5(1993)-244508 where the degree of image processing is changed by correcting the nonlinear functions, whereby the time required to obtain a processed image signal changed with the degree of image processing is shortened and stress on the operator can be lightened. Further since the noise component of the band-limited image signals is removed, the processed image signal is free from noise.
When each of the images represented by the band-limited image signals has picture elements in a number according to the corresponding frequency band, the amount of operation for transformation processing can be reduced and the time required to obtain a processed image signal is further shortened.
Further, when the high frequency component signal (the signal representing information on a high frequency component of the original image signal) has the same number of picture elements as the original image signal, the image processing can be carried out on the original image signal without changing the size of the high frequency component signal, and accordingly the image processing can be carried out more efficiently.
In accordance with a seventh aspect of the present invention, there is provided an image processing method for obtaining a processed image signal from an original image signal representing an original image by carrying out on the original image signal an image processing based on a signal representing information on a high frequency component of the original image signal, the method comprising the steps of
making band-limited image signals from the original image signal,
obtaining a noise signal having the same number of picture element as the original image on the basis of the band-limited image signals,
obtaining a signal representing information on a high frequency component of the original image signal from the band-limited image signals on the basis of a predetermined transformation function, and
removing a noise component from and carrying out the image processing on the original image signal on the basis of the noise signal and the signal representing information on a high frequency component of the original image signal.
That the noise signal has the same number of picture elements as the original image signal means that the image represented by the noise signal is the same in size as the image represented by the original image signal.
The expression xe2x80x9cremoving a noise component from the original image signal on the basis of the noise signalxe2x80x9d means to reduce the noise component included in a reproduced image, and xe2x80x9cremoving a noise component from the original image signal on the basis of the noise signalxe2x80x9d may be performed by, for instance, subtracting from the original image signal its noise component or its noise component multiplied by a coefficient representing a desired noise suppression level. The coefficient may be changed depending on the value of the original image signal.
The xe2x80x9cimage processingxe2x80x9d includes, for instance, a frequency enhancement processing for enhancing a particular frequency component of the original image signal and a dynamic range compression processing for reducing the contrast of the high density range and/or the low density range, thereby narrowing the dynamic range which is the difference between the maximum density and the minimum density of the original image. Further the image processing may comprise both a frequency enhancement processing and a dynamic range compression processing. It is preferred that the xe2x80x9cpredetermined transformation functionxe2x80x9d be a function selected according to the contents of the image processing.
It is preferred that each of the images represented by the band-limited image signals has picture elements in a number according to the corresponding frequency band.
It is preferred that the signal representing information on a high frequency component of the original image signal has the same number of picture elements as the original image signal.
That the signal representing information on a high frequency component of the original image signal has the same number of picture elements as the original image signal means that the image represented by the signal representing information on a high frequency component of the original image signal is the same as the original image in size.
For example, the band-limited image signals may be obtained by carrying out multi-resolution transformation on the original image signal, and the noise signal may be obtained by separating noise components from the band-limited image signals to obtain noise band-limited image signals and carrying out inverse multi-resolution transformation on the noise band-limited image signals.
In this case, the signal representing information on a high frequency component of the original image signal may be obtained by obtaining transformed band-limited image signals by transforming the band-limited image signals by use of said predetermined transformation function and carrying out inverse multi-resolution transformation on the transformed band-limited image signals.
Otherwise the signal representing information on a high frequency component of the original image signal may be obtained by
obtaining transformed noise band-limited image signals by transforming the noise band-limited image signals by use of the predetermined transformation function,
obtaining a transformed noise signal by carrying out inverse multi-resolution transformation on the transformed noise band-limited image signals,
obtaining transformed band-limited image signals by transforming the band-limited image signals by use of the predetermined transformation function,
obtaining a transformed image signal by carrying out inverse multi-resolution transformation on the transformed band-limited image signals, and
subtracting the transformed noise signal from the transformed image signal.
Further, the signal representing information on a high frequency component of the original image signal may be obtained by
obtaining noiseless band-limited image signals by removing noise components from the band-limited image signals,
obtaining transformed noiseless band-limited image signals by transforming the noiseless band-limited image signals by use of said predetermined transformation function, and
carrying out inverse multi-resolution transformation on the transformed noiseless band-limited image signals.
The inverse multi-resolution transformation is a transformation which corresponds to the multi-resolution transformation and is for reconstructing (reversibly or irreversibly) the original signal from the transformed noiseless band-limited image signals.
When the band-limited image signals are obtained by carrying out multi-resolution transformation on the original image signal, Laplacian pyramid decomposition may be employed or a method in which the original image signal is transformed to a plurality of image signals representing the frequency response characteristics of the respective frequency bands by wavelet transformation may be employed. When the band-limited image signals are obtained by the Laplacian pyramid decomposition, Laplacian pyramid reconstruction may be employed as the inverse multi-resolution transformation, and when the band-limited image signals are obtained by the wavelet transformation, inverse wavelet transformation may be employed as the inverse multi-resolution transformation.
The xe2x80x9cnoise band-limited image signalsxe2x80x9d are signals representing only the noise components included in the band-limited image signals.
When the original image signal is transformed to multiple resolution image signals by the Laplacian pyramid decomposition or the wavelet transformation, the image signal in the lowest frequency band represents low frequency information obtained by contracting the original image and is not a band-limited image signal representing the frequency response characteristic. Accordingly, it is preferred that the image signal in the lowest frequency band be not used in the processing or used as 0.
It is preferred that the predetermined transformation function be a nonlinear function.
Further it is preferred that the noise signal be obtained by a filtering processing by an iris filter.
In accordance with an eighth aspect of the present invention, there is provided an image processing system for obtaining a processed image signal from an original image signal representing an original image by carrying out on the original image signal an image processing based on a signal representing information on a high frequency component of the original image signal, the system comprising
a band-limited image signal making means which makes band-limited image signals from the original image signal,
a noise signal obtaining means which obtains a noise signal having the same number of picture element as the original image on the basis of the band-limited image signals,
a high frequency component obtaining means which obtains a signal representing information on a high frequency component of the original image signal from the band-limited image signals on the basis of a predetermined transformation function, and
an image processing means which removes a noise component from and carries out said image processing on the original image signal on the basis of the noise signal and the signal representing information on a high frequency component of the original image signal.
The signal representing information on a high frequency component of the original image signal will be referred to as xe2x80x9cthe high frequency component signalxe2x80x9d, hereinbelow.
It is preferred that the band-limited image signal making aro means makes the band-limited image signals so that each of the images represented by the band-limited image signals has picture elements in a number according to the corresponding frequency band.
It is preferred that the high frequency component obtaining means obtains a high frequency component signal which has the same number of picture elements as the original image signal.
Further, it is preferred that
the band-limited image signal making means makes band-limited image signals by carrying out multi-resolution transformation on the original image signal,
the noise signal obtaining means obtains the noise signal by separating noise components from the band-limited image signals to obtain noise band-limited image signals and carrying out inverse multi-resolution transformation on the noise band-limited image signals, and
the high frequency component obtaining means obtains the signal representing information on a high frequency component of the original image signal by obtaining transformed band-limited image signals by transforming the band-limited image signals by use of said predetermined transformation function and carrying out inverse multi-resolution transformation on the transformed band-limited image signals.
Further, it is preferred that
the band-limited image signal making means makes band-limited image signals by carrying out multi-resolution transformation on the original image signal,
the noise signal obtaining means obtains the noise signal by separating noise components from the band-limited image signals to obtain noise band-limited image signals and carrying out inverse multi-resolution transformation on the noise band-limited image signals, and
the high frequency component obtaining means obtains the signal representing information on a high frequency component of the original image signal by obtaining transformed noise band-limited image signals by transforming the noise band-limited image signals by use of the predetermined transformation function,
obtaining a transformed noise signal by carrying out inverse multi-resolution transformation on the transformed noise band-limited image signals,
obtaining transformed band-limited image signals by transforming the band-limited image signals by use of the predetermined transformation function,
obtaining a transformed image signal by carrying out inverse multi-resolution transformation on the transformed band-limited image signals, and
subtracting the transformed noise signal from the transformed image signal.
Further, it is preferred that
the band-limited image signal making means makes band-limited image signals by carrying out multi-resolution transformation on the original image signal,
the noise signal obtaining means obtains the noise signal by separating noise components from the band-limited image signals to obtain noise band-limited image signals and carrying out inverse multi-resolution transformation on the noise band-limited image signals, and
the high frequency component obtaining means obtains the signal representing information on a high frequency component of the original image signal by obtaining noiseless band-limited image signals by removing noise components from the band-limited image signals, obtaining transformed noiseless band-limited image signals by transforming the noiseless band-limited image signals by use of the predetermined transformation function, and carrying out inverse multi-resolution transformation on the transformed noiseless band-limited image signal.
It is preferred that the noise signal obtaining means and the high frequency component obtaining means obtain the noise signal and the high frequency component signal from the band-limited image signals other than that in the lowest frequency band.
It is preferred that the predetermined transformation function be a nonlinear function.
It is preferred that the image processing comprises a frequency enhancement processing and/or a dynamic range compression processing.
Further it is preferred that the noise signal obtaining means obtains the noise signal on the basis of a filtering processing by an iris filter.
In accordance with a ninth aspect of the present invention, there is provided a computer-readable recording medium loaded with program for causing a computer to perform the image processing method in accordance with the seventh aspect of the present invention.
In accordance also with the seventh to ninth aspects of the present invention, the degree of image processing on the original image signal can be freely changed by only changing the level of the high frequency component signal, and accordingly, the degree of image processing can be easily changed in a shorter time as compared with the method disclosed in Japanese Unexamined Patent Publication No. 5(1993)-244508 where the degree of image processing is changed by correcting the nonlinear functions. Further the noise components of the band-limited image signals according to their frequency bands can be removed. By storing the original image signal and the noise signal and removing the noise component of the original image signal, when the set value of a parameter representing the degree of removing the noise component is changed, on the basis of the stored original signal and noise signal and the changed value of the parameter, the degree of removing the noise signal from the original image signal can be freely changed by only changing the parameter and the level of the noise signal. Accordingly, the degree of image processing and the degree of removing the noise component can be easily changed in a shorter time as compared with the method disclosed in Japanese Unexamined Patent Publication No. 6(1994)-96200, whereby the time required to obtain a processed image signal changed with the degree of image processing and the degree of removing the noise component is shortened and stress on the operator can be lightened.
When each of the images represented by the band-limited image signals has picture elements in a number according to the corresponding frequency band, the amount of operation for the processing can be reduced and the time required to obtain a processed image signal is further shortened.
Further, when the high frequency component signal has the same number of picture elements as the original image signal, the image processing can be carried out on the original image signal without changing the size of the high frequency component signal, and accordingly the image processing can be carried out more efficiently.
In accordance with a tenth aspect of the present invention, there is provided an image processing method for obtaining a processed image signal from an original image signal representing an original image by carrying out a noise removing processing on the original image signal, the method comprising the steps of
making at least one band-limited image signal from the original image signal,
obtaining a noise signal having the same number of picture element as the original image on the basis of the band-limited image signal, and
removing a noise component from the original image signal on the basis of the noise signal.
That the noise signal has the same number of picture elements as the original image signal means that the image represented by the noise signal is the same in size as the image represented by the original image signal.
The expression xe2x80x9cremoving a noise component from the original image signal on the basis of the noise signalxe2x80x9d means to reduce the noise component included in a reproduced image and xe2x80x9cremoving a noise component from the original image signal on the basis of the noise signalxe2x80x9d may be performed by, for instance, subtracting from the original image signal its noise component or its noise component multiplied by a coefficient representing a desired noise suppression level. The coefficient may be changed depending on the value of the original image signal.
For example, the band-limited image signals may be obtained by carrying out multi-resolution transformation on the original image signal, and the noise signal may be obtained by separating noise components from the band-limited image signals to obtain noise band-limited image signals and carrying out inverse multi-resolution transformation on the noise band-limited image signals. The inverse multi-resolution transformation is a transformation which corresponds to the multi-resolution transformation and is for reconstructing (reversibly or irreversibly) the original signal from the transformed band-limited image signals.
When the band-limited image signals are obtained by carrying out multi-resolution transformation on the original image signal, Laplacian pyramid decomposition may be employed or a method in which the original image signal is transformed to a plurality of image signals representing the frequency response characteristics of the respective frequency bands by wavelet transformation may be employed. When the band-limited image signals are obtained by the Laplacian pyramid decomposition, Laplacian pyramid reconstruction may be employed as the inverse multi-resolution transformation, and when the band-limited image signals are obtained by the wavelet transformation, inverse wavelet transformation may be employed as the inverse multi-resolution transformation.
The xe2x80x9cnoise band-limited image signalsxe2x80x9d are signals representing only the noise components included in the band-limited image signals.
It is preferred that the noise signal be obtained by a filtering processing by an iris filter.
Further the noise signal may be obtained on the basis of picture element vectors calculated for picture elements of the image represented by the band-limited image signal.
The picture element vector represents the inclination and the direction of inclination of the value of an object picture element (a picture element of the image represented by a band-limited image signal). For example, the differences between the value of the object picture element and picture elements near the object picture element in a plurality of directions (when a plurality of picture elements are taken in one direction, the difference between the value of the object picture element and the average of the values of the picture elements near the object picture element) are calculated and the direction in which the difference is maximized or minimized is determined. Then the picture element vector is calculated on the basis of the direction in which the difference is maximized or minimized and the value of the difference.
When the picture element vector is calculated on the basis of the direction in which the difference is maximized, the picture element vector represents the direction of signal gradient, and when the picture element vector is calculated on the basis of the direction in which the difference is minimized, the picture element vector represents the direction of equi-signal line. When the picture element vector is obtained in the direction of signal gradient and the length of the vector represents the difference in value between the object picture element and a picture element near the object picture element, the probability that the picture element is on an edge portion is higher as the length of the vector increases and the probability that the picture element is on a flat portion is higher as the length of the vector decreases. When the picture element vector is obtained in the direction of signal gradient and the length of the vector represents the reciprocal of the difference in value between the object picture element and a picture element near the object picture element, the probability that the picture element is on an edge portion is higher as the length of the vector decreases and the probability that the picture element is on a flat portion is higher as the length of the vector increases.
When the picture element vector is obtained in the direction of equi-signal line and the length of the vector represents the difference in value between the object picture element and a picture element near the object picture element, the probability that the picture element is on an edge portion is higher as the length of the vector decreases and the probability that the picture element is on a flat portion is higher as the length of the vector increases. When the picture element vector is obtained in the direction of equi-signal line and the length of the vector represents the reciprocal of the difference in value between the object picture element and a picture element near the object picture element, the probability that the picture element is on an edge portion is higher as the length of the vector increases and the probability that the picture element is on a flat portion is higher as the length of the vector decreases.
As the direction of the picture element vector, the direction in which the difference is maximum and the direction in which the difference is second maximum may be both obtained. In this case, the picture element vector comprises the two vectors.
When the picture element vector is obtained in the direction of the equi-signal line for a certain object picture element and the length of the vector represents the reciprocal of the difference, the probability that the picture element is on an edge portion is higher as the length of the vector increases and the probability that the picture element is on a flat (in density) portion is higher as the length of the vector decreases as described. In this case, the picture element may be considered to be noise on the flat portion.
By determining whether the picture element is on a flat portion or on an edge portion on the basis of the direction and/or the length of the picture element vector, the noise signal may be obtained from the band-limited image signals (the noise component of the band-limited image signals may be separated from the band-limited image signals) on the basis of the result of the determination.
For example, by separating the noise component and the edge component of the band-limited image signal from each other on the basis of the lengths of the picture element vectors, carrying out a noise smoothing processing and/or an edge enhancing processing on the band-limited image signal, a processed band-limited image signal may be obtained and the noise signal included in the band-limited image signal before the noise smoothing processing may be obtained by the use of the processed band-limited image signal. The noise smoothing processing is a processing in which the values of the picture elements of the noise component are reduced and the edge enhancing processing is a processing in which the values of the picture elements of the edge component are increased.
When the noise signal is obtained on the basis of the picture element vector, it is preferred that the picture element vectors of picture elements near each object picture element (surrounding picture element vectors) be calculated and the noise signal be obtained on the basis of also the surrounding picture element vectors.
It is preferred that a picture element vector calculated for a given picture element of an image represented by a band-limited image signal in a given frequency band be corrected on the basis of the picture element vector of the corresponding picture element (the picture element corresponding to the given picture element) of an image represented by a band-limited image signal in a frequency band lower than the given frequency band, and said noise signal be obtained on the basis of the corrected picture element vector.
For example, the picture element vector calculated for the given picture element may be corrected so that the direction of the picture element vector of the given picture element conforms to that of the corresponding picture element of the image represented by the lower frequency band-limited image signal.
It is further preferred that whether a picture element vector calculated for a given picture element of an image represented by a band-limited image signal in a given frequency band is to be corrected be determined on the basis of dispersion of the band-limited image signal in a predetermined region including the given picture element, and when it is determined that the picture element vector calculated for the given picture element is to be corrected, the picture element vector be corrected on the basis of the picture element vector of the corresponding picture element (the picture element corresponding to the given picture element) of an image represented by a band-limited image signal in a frequency band lower than the given frequency band, and said noise signal be obtained on the basis of the corrected picture element vector.
In place of the dispersion, the difference between the value of the given picture element and picture elements near the given picture element which is used in calculating the picture element vector for the given picture element may be employed. The difference may be the sum of the differences between the given picture element and a plurality of picture elements near the given picture element or the average of the differences.
For example, when the dispersion of the band-limited image signal in the predetermined region is smaller than the dispersion in other regions, the predetermined region may be considered to be a flat portion and accordingly the picture element vector for the given picture element need not be corrected and otherwise the picture element vector should be corrected.
When the picture element vector for a given picture element is determined to be corrected, the picture element vectors for a plurality of picture elements around the given picture element may be corrected.
Further, it is possible to obtain the noise signal on the basis of a smoothed band-limited image signal obtained by smoothing the band-limited image signal on the basis of the picture element vectors.
In this case, it is preferred that the smoothed band-limited image signal be obtained by smoothing the band-limited image signal on the basis of the picture element vectors corrected in the manner described above.
The expression xe2x80x9csmoothing the band-limited image signal on the basis of the picture element vectorsxe2x80x9d means to smooth the band-limited image signal on the basis of the picture element vectors (especially on the basis of the directions of the picture element vectors) so that the noise contained in the edge component is suppressed with the edge component reserved. For example, when the picture element vector is in the direction of the equi-signal line, the band-limited image signal may be smoothed by the use of picture elements which are in the direction of the picture element vector from the object picture element for the picture element vector and picture elements which are in the direction opposite to the direction of the picture element vector from the object picture element. The smoothing may be effected by obtaining the average of the values of picture elements which are in the direction of the picture element vector from the object picture element for the picture element vector, or by smoothing the band-limited image signal by the use of a smoothing filter.
The noise signal may be obtained on the basis of the smoothed band-limited image signal in any manner provided that the noise component of the band-limited image signal can be separated from the band-limited image signal and the noise signal can be obtained. For example, a noise signal (a noise band-limited image signal) may be obtained by subtracting smoothed band-limited image signal from the band-limited image signal. Further a noise signal may be obtained by separating the noise component and the edge component of the smoothed band-limited image signal from each other on the basis of the lengths of the picture element vectors, carrying out a noise smoothing processing and/or an edge enhancing processing on the smoothed band-limited image signal to obtain a processed band-limited image signal, and obtaining the noise signal contained in the band-limited image signal before the noise smoothing processing by the use of the processed band-limited image signal. The noise smoothing processing is a processing in which the values of the picture elements of the noise component are reduced and the edge enhancing processing is a processing in which the values of the picture elements of the edge component are increased.
It is preferred that the original image signal and the noise signal be stored and the noise component of the original image signal be removed, when the set value of a parameter representing the degree of removing the noise component is changed, on the basis of the stored original signal and noise signal and the changed value of the parameter.
In accordance with an eleventh aspect of the present invention, there is provided an image processing system for obtaining a processed image signal from an original image signal representing an original image by carrying out a noise removing processing on the original image signal, the system comprising,
a band-limited image signal making means which makes at least one band-limited image signal from the original image signal,
a noise signal obtaining means which obtains a noise signal having the same number of picture element as the original image on the basis of the band-limited image signal, and
a noise removing means which removes a noise component from the original image signal on the basis of the noise signal.
For example, the band-limited image signal making means may make the band-limited image signal by carrying out multi-resolution transformation on the original image signal, and the noise signal obtaining means may obtain the noise signal by separating the noise component from the band-limited image signal to obtain noise band-limited image signal and carrying out inverse multi-resolution transformation on the noise band-limited image.
The multi-resolution transformation may comprise, for instance, Laplacian pyramid decomposition or a wavelet transformation.
It is preferred that the noise signal obtaining means obtains the noise signal by a filtering processing by an iris filter.
Further it is preferred that the noise signal making means comprises a picture element vector obtaining means which calculates picture element vectors for picture elements of the image represented by the band-limited image signal and obtains the noise signal on the basis of the picture element vectors.
When the noise signal is obtained on the basis of the picture element vector, it is preferred that the noise signal obtaining means calculates the picture element vectors of picture elements near each picture element and obtains the noise signal on the basis of also the picture element vectors of picture elements near the object picture element.
Further, when the noise signal is obtained on the basis of the picture element vector, it is preferred that the noise signal obtaining means be provided with a correcting means which corrects a picture element vector calculated for a given picture element of an image represented by a band-limited image signal in a given frequency band on the basis of the picture element vector of the corresponding picture element of an image represented by a band-limited image signal in a frequency band lower than the given frequency band, and obtains the noise signal on the basis of the corrected picture element vector.
Further, when the noise signal is obtained on the basis of the picture element vector, it is preferred that the noise signal obtaining means be further provided with a dispersion calculating means which calculates dispersion of the band-limited image signal in a predetermined region including a given picture element of an image represented by a band-limited image signal in a given frequency band, a determining means which determines whether a picture element vector calculated for the given picture element is to be corrected on the basis of the dispersion, and a correcting means which, when it is determined that the picture element vector calculated for the given picture element is to be corrected, corrects the picture element vector on the basis of the picture element vector of the corresponding picture element of an image represented by a band-limited image signal in a frequency band lower than the given frequency band, and obtains the noise signal on the basis of the corrected picture element vector.
Further, when the noise signal is obtained on the basis of the picture element vector, the noise signal obtaining means may further comprise a smoothing means which smoothes the band-limited image signal on the basis of the picture element vectors to obtain a smoothed band-limited image signal and may obtain the noise signal on the basis of the smoothed band-limited image signal.
In this case, it is preferred that the smoothing means obtains the smoothed band-limited image signal on the basis of the picture element vectors corrected in the manner described above.
It is preferred that the image processing system further comprises a first memory means which stores the original image signal, a second memory means which stores the noise signal obtained by the noise signal obtaining means, and a parameter setting means which sets the value of a parameter representing the degree of removing the noise component to the noise removing means, and the noise removing means reads out the original image signal and the noise signal from the first and second memory means and removes the noise component of the original image signal, when the set value of the parameter representing the degree of removing the noise component is changed, on the basis of the stored original signal and noise signal and the changed value of the parameter.
In accordance with a twelfth aspect of the present invention, there is provided a computer-readable recording medium loaded with program for causing a computer to perform the image processing method in accordance with the tenth aspect of the present invention.
In accordance with the tenth to twelfth aspects of the present invention, the noise components of the band-limited image signals can be removed according to their frequency bands.
By storing the original image signal and the noise signal and removing the noise component of the original image signal, when the set value of a parameter representing the degree of removing the noise component is changed,on the basis of the stored original signal and noise signal and the changed value of the parameter, the degree of removing the noise signal from the original image signal can be freely changed by only changing the parameter and the level of the noise signal. Accordingly, the degree of removing the noise component can be easily changed in a shorter time as compared with the method disclosed in Japanese Unexamined Patent Publication No. 6(1994)-96200, whereby the time required to obtain a processed image signal changed with the degree of removing the noise component is shortened and stress on the operator can be lightened.
The band-limited image signal may be made in various ways. For example, the band-limited image signal may be of the same size as the original image signal. In this case, the noise signal may be obtained by smoothing the original image with different sizes of masks to obtain a plurality of band-limited image signals of the same size as the original image signal and adding up noise band-limited image signals separated from the respective band-limited image signals. Of course, the band-limited image signal may be obtained by said multi-resolution transformation as described above.
Further, the noise signal may be obtained in various ways. For example, the noise signal may be obtained by calculating the picture element vector for each picture element of band-limited image represented by a band-limited image signal and separating the noise component (noise signal) from the band-limited image signal on the basis of the picture element vectors. Though depending on whether the picture element vectors are obtained in the direction of the equi-signal line or the direction of signal gradient and whether the lengths of the vector represent the differences or the reciprocals of the differences, the picture element vectors are long in the edge portion and short in the flat portion (noise portion) when the picture element vectors are obtained in the direction of equi-signal line and the lengths of the vectors represent the reciprocals of the differences. In this case, the noise component of the band-limited image signal can be separated according to the length of the picture element vector. For example, the noise signal can be obtained by carrying out a smoothing processing for reducing the values of the picture elements of the noise component on the band-limited image signal, and separating the noise component from the band-limited image signal on the basis of the smoothed band-limited image signal.
When the picture element vectors are obtained in the direction of equi-signal line and the lengths of the vectors represent the reciprocals of the differences, a picture element whose picture element vector is relatively short may be generally considered to be in a flat portion, i.e., to be noise, however there still remains a possibility that the picture element is in a small edge portion in the image. When the picture element is in an edge portion, the picture element vectors of picture elements near the picture element are directed in the same direction as the picture element vector of the picture element. On the other hand, when the picture element is noise, the picture element vectors of picture elements near the picture element are directed in random directions. Accordingly, by taking into account the picture element vectors of picture elements near a given picture element when determining whether the given picture element is in an edge portion or noise, the determination can be more precise.
Though a relatively large edge in an original image is held in an image represented by a band-limited image signal in a relatively low frequency band, noise becomes smaller as frequency band becomes lower. Accordingly, when a picture element vector calculated for a given picture element of an image represented by a band-limited image signal in a given frequency band is corrected on the basis of the picture element vector of the corresponding picture element of an image represented by a band-limited image signal in a frequency band lower than the given frequency band, the corrected picture element vector more precisely reflects whether the picture element is in the edge portion or the flat portion, whereby the noise component and the edge component can be separated more precisely.
When the original image signal is transformed into multiple resolution image signals, band-limited image signals in higher frequency bands hold detailed edge information, those in intermediate frequency bands hold intermediate edge to information and those in lower frequency bands hold rough edge information. Generally energy of an image is reduced as the frequency band becomes higher whereas energy of noise does not depend upon the frequency band. Accordingly, the S/N ratio becomes higher as the frequency band becomes lower. In a part of the original image free from noise (e.g., as indicated at (a) in FIG. 59), the band-limited image signal in any frequency band has a value only at an edge portion as indicated at (b) to (d) in FIG. 59. Accordingly, when the dispersion of picture elements in the predetermined region including a given object picture element in an image represented by a relatively high frequency band-limited image signal is small, the given object picture element may be considered to be in a flat portion without referring to the picture element vector of the corresponding picture element in an image represented by a lower frequency band-limited image signal.
To the contrast, in a part of the original image including noise (e.g., as indicated at (a) in FIG. 60), directions of the picture element vectors are disturbed by the noise and the dispersion becomes larger in images represented by high frequency band-limited image signals as indicated at (b) in FIG. 60, whereas influence of the noise becomes weaker and the dispersion becomes smaller as the frequency band becomes lower as indicated at (c) and (d) in FIG. 60. Accordingly, when the dispersion of picture elements in the predetermined region including a given object picture element in an image represented by a relatively high frequency band-limited image signal is large, it is difficult to accurately determine where the given object picture element is in a flat portion or an edge portion without referring to the picture element vector of the corresponding picture element in an image represented by a lower frequency band-limited image signal. Accordingly, when the dispersion of picture elements in the predetermined region including a given object picture element in an image represented by a relatively high frequency band-limited image signal is large, by correcting the picture element vector calculated for the given picture element to conform to the picture element vector calculated for the corresponding picture element in an image represented by a lower frequency band-limited image signal, whether the given picture element is in an edge portion or in a flat portion can be more precisely determined according to the corrected picture element vector.
When noise is included in an image, the edge component in the image also includes noise. When the band-limited image signal is smoothed on the basis of (the directions) of the (corrected) picture element vectors and the noise signal is obtained on the basis of the smoothed band-limited image signal in place of the picture element vectors, the noise component in the edge can be extracted without losing the edge component and the noise in the flat portion can be also extracted. Accordingly, the noise on the edge can be made less conspicuous and also the noise on the flat portion can be made less conspicuous.
Further when a noise signal is obtained by separating the noise component and the edge component of the smoothed band-limited image signal from each other on the basis of the lengths of the picture element vectors, carrying out a noise smoothing processing and/or an edge enhancing processing on the smoothed band-limited image signal to obtain a processed band-limited image signal, and obtaining the noise signal contained in the band-limited image signal before the noise smoothing processing by the use of the processed band-limited image signal, the edge can be enhanced without making the noise on the edge more conspicuous and the noise in the flat portion can be more suppressed, whereby the image can be reproduced in a higher quality.